Clutter is a Microsoft Outlook feature that attempts to help you filter low-priority email, saving time for your most important messages. The email server keeps track of the email you read and the ones you don't. Clutter is automatic. As new email comes in, it takes messages you're most likely to ignore and puts them into the "Clutter" folder. The more you use it, the better it gets. And if you find Clutter isn't for you, you can turn it off.
If you wish to use Clutter it is important to check your Clutter folder often while it is learning what emails are likely to be less important. There is also an option to disable Clutter if you don’t wish to use it. More information, including how to train Clutter to make more appropriate choices is available at https://support.office.com/en-in/article/Use-Clutter-to-sort-low-priority-messages-in-Outlook-2016-for-Windows-7b50c5db-7704-4e55-8a1b-dfc7bf1eafa0. To disable Clutter in Outlook right-click on Clutter and select Manage Clutter.